Nassau Groupers: Morphology and Life History
As part of Green Reef's Nassau Grouper Research and Advocacy Campaign, this week's Reef Brief highlights the morphology and life history of this important commercial species. Watch for next week's Reef Brief article, which will discuss why Nassau Groupers are especially vulnerable to over fishing and therefore may need special management measures.
Nassau Groupers exhibit no sexual dimorphism in body shape or color, so it is impossible from a glance to determine if a particular fish is male or female. They possess some degree of protogynous hermaphroditism, which means they change from female to male when they are between 300 and 800 millimeters long. Adult Nassau Groupers generally eat a diet of mainly fish (parrotfish, wrasses, damselfish, squirrelfish, snapper and grunts). Juveniles, on the other hand, eat a diet composed mainly of crustaceans (crab, stomatopod, hermit crab, panulirid lobster, and caridean and panaid shrimp). These juvenile groupers are usually found around coral clumps covered with macroalgae and over sea grass beds.
Nassau Groupers, as are many other type of groupers and coral reef fishes, are solitary during most of the year and then come together to spawn in large aggregations. These aggregation banks are site specific, usually in 20-40 meters of water at specific locations at the outer reef shelf where temperatures are between 25-26º Celcius. In Belize, Nassau Groupers have historically aggregated around the full moons of December and January in groups that can contain upwards of several thousand individuals. During these aggregations, the female groupers produce planktonic eggs that are fertilized externally. The reproductive success of the eggs is thought to be affected by predators and currents.
As mentioned in last week's Reef Brief article, Nassau Groupers are an important commercial species in Belize. However, there are several factors which makes the Nassau Grouper especially vulnerable to over fishing. Nassau Groupers come together in large groups, called spawning aggregations, during the full moons of December and January. Fishermen are familiar with these cycles and find that the best time to catch this species is when they are in these large spawning groups. The spawning aggregation sites are generally in the same location year after year which makes it easy for fisherman to return to the sites. The fish also take a long time to reach maturity, which means that if a large number of mature adults are fished from the population one year, it will take a long time before the population can return to normal. The size of the population can also be negatively affected if the fish are caught before they have had the chance to spawn. These characteristics combined with the relatively strong demand for grouper fillet and roe (eggs) suggest that some measures need to be taken to ensure that this fishery does not collapse.
This situation is not one that exists just in Belize. Populations of Nassau Groupers in the Caribbean are reported to be declining and some countries (Bermuda, U.S. Virgin Islands and Puerto Rico) have even gone as far as to close this fishery in order to protect them from extinction. Several countries, including the Dominican Republic, have specific closed seasons for the Nassau Grouper in an effort to protect the species when they are most susceptible to fishing pressures. Other countries, including Mexico and the Cayman Islands have instituted gear restrictions. Belize has made several efforts at conserving this species, including establishing marine reserves at some of the spawning aggregations, but due to the continued decline in annual catches, it is clear that additional measures may be needed to protect this species and the fishery that it supports.
Press Release for UNDP GEF funds
The Nassau Grouper has been reported to be the second most important commercial fishery in Belize but is widely believed to be at risk of collapsing due to extensive over-fishing of the species' spawning aggregations. The Nassau Grouper is particularly vulnerable to over-fishing pressures due to its life history characteristics. Large numbers of Nassau Groupers aggregate during the full moons of December and January to spawn. These spawning aggregations are often targeted by fisherman who are familiar with the sites and are able to capture large numbers of the fish, often before they have had a chance to reproduce.
In order to better understand the state of the Nassau Grouper populations in Belize, Green Reef will be coordinating video surveys at nine sites during the spawning period in January. Several agencies and organizations have been extremely generous in donating their time and resources to furthering this important stock assessment. Green Reef wishes to acknowledge the contributions made by the Belize Fisheries Department, Bacalar Chico National Park and Marine Reserve, Hol Chan Marine Reserve, The Nature Conservancy, Wildlife Conservation Society, Toledo Institute for Development and Environment (TIDE), the Belize Audubon Society, Ramon's Village and Amigos del Mar.
For more information on the work that Green Reef is conducting, please contact them at E-mail: - [email protected] or 026-3254 ext 243.
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